The Net Positive Suction Head (NPSH) margin is a crucial factor that is commonly overlooked while selecting a pump. It is the difference between the NPSH available (NPSHa) at the pump’s inlet and the NPSH required (NPSHr) by the pump to operate without cavitation. Cavitation is the formation of bubbles at the pump inlet, followed by their sudden collapse, which can cause permanent damage. The NPSH margin value must be positive to avoid cavitation.
Pump designers use NPSH to ensure that pumps will operate without internal damage caused by cavitation under all specified operating conditions.
NPSH Available (NPSHA): The absolute pressure at the suction port of the pump.
NPSH Required (NPSHR): The minimum pressure required at the suction port of the pump to keep the pump from cavitating.
NPSH or Net Positive Suction Head is a very important part of a pumping system. The systems NPSHA must be higher than the pump’s NPSHR in order for proper pump performance and to eliminate the risk of cavitation, which can damage a pump in short order and shut down operations that depend on that pump.
How does NPSH relate to Pump Operation?
Pumps work by creating a pressure differential between the inlet and the outlet—the high pressure at the inlet causes fluid to flow towards the low pressure at the outlet. As the fluid flows through the pump, it experiences steadily decreasing pressure along the flow path. If the pressure at the inlet falls below the vapor pressure of the fluid, bubbles will form at the inlet.
These bubbles collapse rapidly inside the pump as they move towards the outlet. This cavitation causes the pump to operate noisily, making it sound like something like gravel in a concrete mixer. The bubbles in the fluid also reduce the capacity of the pump. Most importantly, cavitation removes small spots of material from the inside walls of the pump, pitting the walls and causing permanent damage to the pump.
How to Use Net Positive Suction Head to Select a Pump
NPHSr, the NPSH required for the proper operation of the pump, depends on the pump design, the rotational speed of its impellers, and its age. Pitting also depends on the hardness of the material that the pump is made from, which makes that a factor as well.
NPHSa—the NPSH available at the inlet—depends entirely on the operating environment, including the temperature of the fluid being pumped. As explained above, it is crucial to maintain a positive margin between NPHSa and NPHSr.
As a general rule, make sure that NPHSr is less than NPHSa by the larger of 5 feet or 10% of NPHSa. For example, if NPHSr is 10 feet, NPHSa must be at least 15 feet.
NPHSa calculations should consider the temperature of the fluid and the distance between the pump and the suction source. Cavitation occurs more readily at higher temperatures since vapor pressure increases with temperature. If the suction source is below the pump, the vertical distance between the two reduces the NPHSa at the pump’s inlet.
It is necessary to measure pressure accurately with pressure gauges to ensure selection of the right pump for the job.
Gauge Pressure Equation
Pressure gauges measure pressure relative to atmospheric pressure. The readout of a pressure gauge is called gauge pressure. Atmospheric pressure, nominally 14.7 psi at sea level, is added to gauge pressure to get absolute pressure. Therefore:
- Absolute Pressure = Gauge Pressure + Atmospheric Pressure
Vapor pressure of a fluid is expressed as absolute pressure. So absolute pressure is used for calculating NPSHa using the formula:
NPSHa = Ha – Hvp.
- Ha = absolute pressure of the fluid at the inlet of the pump
- Hvp = absolute vapor pressure of the fluid at the operating temperature
Using this formula ensures selection of a pump whose NPSHr is less than the greater of 10% of NPHSa and 5 feet.
**This is only a brief summary of THIS paper put together by MasoSine Process Pumps
**Learn More About MasoSine Pumps and Their Low NPSHR HERE…
Pump Selection Assistance from Springer Pumps
Springer Pumps features fluid handling solutions for a broad range of industries. Our pumps are used in a variety of applications ranging from unloading rail cars to pumping liquids, pastes, and gels. For more information about pump selection or calculating NPSH values for your application, check out our NPSH guide or contact us.